Fixed heading numbers.

Some sections jumped from level 2 to level 4, for example.

Also played around with generating book to HTML. I like it, but
it needs some work. The Calico table of content generator cannot
deal with different headers with the same name, for example.

I want to go to HTML so I can use Bokeh to have interactive plots.
But that means no more PDF, because Bokeh will not export to
PDF.

code/secnum.py

@@ -24,7 +24,7 @@ Thanks also to:
 
 import io
 import os
-import urllib2
+import urllib3
 
 from IPython.display import display_html, display_javascript
 
@@ -42,7 +42,7 @@ def download(fname, redownload=False):
     url = 'https://raw.github.com/dpsanders/ipython_extensions/master/section_numbering/' + fname
     print("Downloading %s to %s" % (url, dest))
     
-    filein  = urllib2.urlopen(url)
+    filein  = urllib3.urlopen(url)
     fileout = open(dest, "wb")
     chunk = filein.read(1024)
     while chunk: